This invention relates to maintenance-free rechargeable sealed lead-acid cells (batteries) of the absorbed electrolyte type and in particular to a method for producing such batteries using in situ electrochemical formation of the electrode plates.
The basic sealed gas recombining lead-acid battery of the absorbed electrolyte type is taught in McCelland et al U.S. Pat. No. 3,862,861. That patent teaches the preferred use of a separator material made from microfiber glass matting highly absorptive of the electrolyte. The fiber diameter of the glass is taught to be in the range of 0.2 to 10 microns, with a surface area of approximately 0.1 to 20 m.sup.2 /g of silica and a porosity as high as 85-95 percent. In commercial practice batteries of this type have employed ultrafine glass fiber nonwoven mats composed of different fiber diameter components, with a corresponding surface area in the range of 2.0-2.4 m.sup.2 /g. Typical separator materials of this type are illustrated in published U.K. Patent Application No. 2051464A (see scanning electron photomicrographs of FIGS. 3 and 4). Sealed recombining batteries of this type in both parallel plate prismatic and spiral wound configurations have enjoyed considerable commercial success.
Two basic methods have typically been used to electrochemically form the plates of sealed gas recombining lead-acid batteries. During the formation step typically the lead sulfate and lead oxide in the positive plate are oxidized to form lead dioxide, and in the negative plate the lead sulfate and lead oxide are reduced to spongy lead. In the first method which is typically used to form flat plates for stacking into prismatic configurations, the plates are pre-formed e.g., tank formed, subsequently assembled with interleaved ultrafine glass fiber mats of the aforementioned type, inserted into a container with the plates and separators compressed together, and then electrolyte is added and the batteries sealed.
In the second method, such as is disclosed in the aforementioned McCelland et al patent, unformed plates are assembled with interleaved highly absorbent separators, inserted into the container with the plates and separators existing under mutual compression, electrolyte is then added, and then the plates are electrochemically formed in situ. The formation electrolyte also serves as the final electrolyte. This method is particularly useful for producing cells having plates of continuous lengths wound or folded together. For instance, it is not considered possible to wind pre-formed active lead plates into a spirally wound configuration since the plates are stiff, will crack and otherwise lose their integrity upon winding.
Non-sealed lead-acid batteries have also been formed by the above methods and also by a fill and dump method. In this latter method the plates with interposed separator are formed in the container using low specific gravity acid which is subsequently dumped and replaced with higher gravity acid.
All commercial sealed recombining cells made by the in situ formation process have, to Applicants' knowledge, employed microfine glass fiber mat separators with a surface area in the range from 2.0-2.4 m.sup.2 /g, and a porosity in the range of about 85 to about 95 percent. This high void volume and high surface make it possible for the separator to absorb relatively large amounts of acid while still retaining a substantial void volume sufficient for oxygen to be transported from the positive to the negative electrode plates during overcharge where it is recombined. However, because of this large separator surface area and high affinity of the glass for sulfuric acid, it has been difficult particularly in cells having extended width plates to obtain a cell where acid is distributed evenly over the total separator volume, and where an adequate proportion of the acid is partitioned into the plates. The relatively low acid level within the plates retards the high-rate performance of the cells (where capacity is limited by the amount of acid within the pores of the active material). Uneven distribution of acid within the separator envelope creates areas in the cell where the specific gravity of the electrolyte is low, or where dry bands are formed, and normally low corrosion rates are greatly accelerated.
Additional relevant art includes copending commonly assigned application U.S. Ser. No. 375,503, filed May 6, 1982, and aforementioned U.K. Patent Application No. GB2051464A published Jan. 14, 1981.
The present invention has as its primary object the provision of a sealed recombining lead-acid battery, produced using an in situ formation process and having improved high rate performance particularly at low temperatures, without compromising oxygen recombination efficiency upon charge of the battery. It is another object to produce the aforementioned battery by a method in which the separator component is chosen to produce a more homogeneous distribution of electrolyte through the total separator volume, and a more favorable partitioning of the electrolyte between the separators and plates pressed thereagainst, to enhance the formation process and subsequent performance of the battery.